A typical turbo shaft engine has a mechanical torque sensing device that drives a cockpit indicator so that the pilot or operator can know the power output of the engine. Torque is a critical parameter monitored by pilots and engine operators to control the aircraft or other engines and prevent damage to other drive train components. Most torque meters actually measure the twist in a drive shaft within the engine for torque indication. The accuracy of these torque measurements is affected by the shaft material properties, the temperature of the shaft, the frictional components that support the shaft and the torsional creep of the shaft itself. In addition, deficiencies in the accuracy, resolution, environmental response of the transducer, signal conditioning and computations have a large effect on the measurement accuracy. The cumulative effect of such deficiencies often is a torque indication that is unsatisfactory for smooth, safe and accurately reliable engine control.
One of the means to improve torque accuracy involves characterizing each torque shaft individually against a reference torque measurement system by entering the shaft-specific data into an electronic engine controller. With this shaft-specific data, the electronic engine controller can correct torque sensor signals to account for shaft material properties and operating conditions. Work has been done on improving the materials used for building torque shafts to achieve more uniform material characteristics. Low friction sleeves and bushings have been installed between reference shafts and load-carrying shafts to improve torque meter performance.
Another means for achieving accurate torque reading utilizes algorithms developed to adjust the torque readings to account for temperature variations in the torque meter shaft. Because a typical turbine engine is used to produce varying power output, the internal temperature of the engine changes constantly. This change in temperature causes a change in temperature of the torque meter. As is well-known, when a metal is subjected to changes in temperature, its material properties change which allows the metal to twist a different amount in response to the same applied torque. Corrective algorithms neutralize the effects of the temperature variations. But the use of corrective algorithms necessitates the added complexity of taking shaft temperature measurement or generating a synthesized (i.e. approximated) shaft temperatures and, as a result, reduces system reliability.
In providing torque indication for a helicopter engine, a single pressure tap in front of the power turbine has been used. But this positioning of the single tap cannot account for exhaust system losses or the effects of the dynamics of the helicopter, such as changes in the helicopter speed and the flight attitude that affect the backpressure to the engine. All these aspects tend to reduce the accuracy of the torque measurements.
Because of the general unreliability of many torque sensors, synthesized torque signals are often used by engine control systems as a backup torque signal. Synthesized torque signals are generated by using other engine parameters such as compressor discharge pressure, gas generator speed, turbine inlet temperature or combinations of these and pre-established engine characteristics. Such synthesized torque signals can give an approximate engine torque indications but are plagued with inaccuracies due to off-design operation, engine deterioration from wear and tear and even bleed air extraction in many turbine applications.